Glasses, plastics, or single crystals such as lithium niobate (LiNbO3), have heretofore been used as materials of optical components, such as lenses, for use in optical devices such as optical pickups, as described in Patent Document 1 and Patent Document 2.
Glasses and plastics have high light transmittance and are easily processed into desired shapes. Thus, glasses and plastics have been used in optical components, primarily in lenses. LiNbO3 single crystals are used in optical components, primarily in optical waveguides, in which the electro-optical characteristics and the birefringence of the LiNbO3 single crystals are utilized. In optical devices, such as optical pickups, including such optical components, there has been a demand for smaller optical devices having lower profiles.
However, existing glasses and plastics have refractive indices of 2.00 or less (determined at a wavelength of 633 nm which, unless otherwise specified, the refractive index used herein is determined at that wavelength). In optical components and optical devices formed of glasses or plastics, therefore, the reduction in size and thickness is limited by these low refractive indices. Furthermore, plastics disadvantageously have poor moisture resistance and sometimes exhibit birefringence. Thus, it is difficult to allow incident light to pass efficiently through plastics and to converge the incident light.
On the other hand, the LiNbO3 single crystal has a refractive index as high as 2.3, but it exhibits birefringence. The LiNbO3 single crystal is therefore difficult to use in optical components such as lenses and has limited applications.
Ba(Mg, Ta)O3 and Ba(Zn, Ta)O3 translucent ceramics are known to exhibit no birefringence and have excellent optical properties, as described in Patent Document 3. These translucent ceramics have refractive indices of at least 2.01.
Recently, a high anomalous dispersion Δθg,F, which is one of measures of optical properties, is sometimes required. It will be further described below. Having an anomalous dispersion means that the wavelength dispersion characteristic is different from those of common optical glasses. A high anomalous dispersion Δθg,F is useful for the correction of chromatic aberration. The anomalous dispersion is represented by negative values in the present specification and a high anomalous dispersion is represented by a large absolute value.
Ba(Mg, Ta)O3 and Ba(Zn, Ta)O3 translucent ceramics disclosed in Patent Document 3 have perovskite structures represented by a general formula ABO3, and in particular, have complex-perovskite structures in which the B-site is composed of at least two elements. More specifically, the complex-perovskite structures principally include a bivalent metallic element composed of Mg and/or Zn and a pentavalent metallic element composed of Ta and/or Nb, at a molar ratio of about 1:2. Thus, the complex-perovskite structures substantially have electroneutrality. Furthermore, optical properties such as the refractive index and the Abbe number can be changed by replacing Mg, Ta and/or Zn of the B-site element with a quadrivalent element such as Sn or Zr.
However, the translucent ceramic described in Patent Document 3 disadvantageously has a low anomalous dispersion Δθg,F. For example, Ba{(Sn, Zr)Mg, Ta}O3, Ba(Zr, Zn, Ta)O3 and Ba{(Sn, Zr)Mg, Nb}O3 have Δθg,Fs as low as −0.013, −0.006 and −0.000, respectively.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-127078.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-244865.
Patent Document 3: International Publication WO 02/49984.